The present invention relates to a modified cellulose with a low degree of substitution, and to a process for manufacturing this cellulose.
The invention relates more particularly to a cellulose containing relatively little substitution, which has improved reactivity and a low degree of crystallization.
Cellulose is a natural polymer present in abundance in nature, in a very wide variety of forms, such as plants (wood, cotton, flax, etc.) or even animals (molluscs, etc.). This natural polymer has been used for a very long time, in particular for the manufacture of paper, textile fibres, plastics or the like.
Modified forms such as cellulose esters or cellulose ethers have also been synthesized. The cellulose ester most commonly used is cellulose acetate, which is used as a plastic material for the manufacture of moulded or extruded articles, in the form of fibres or yarns for the textiles sector or in cigarette filters, for example.
Cellulose ethers are cellulose derivatives which are generally soluble in a solvent such as water or an organic solvent. Cellulose ethers are used in particular as thickeners, agents for controlling the fluidity or viscosity of a medium, and dispersing agents. They can also be used for the formation of colloids or liquid crystals, or as a matrix for the manufacture of films.
Relatively little commercial development of cellulose derivatives with very advantageous properties is performed, with the exception of only a few derivatives such as cellulose acetate or carboxymethyl cellulose.
The reason for this is that cellulose derivatives are obtained by reaction of a compound of substitution on the hydroxyl functions of the cellulose structure. In order to obtain a product which is homogeneous and in particular soluble in solvents, it is necessary to substitute all or most of the hydroxyl groups of the cellulose, and if only some of these groups need to be substituted, the distribution of the substituents should be uniform throughout the cellulose.
However, cellulose has a structure comprising crystalline portions and amorphous portions. Consequently, without treating it beforehand, it is difficult to substitute the hydroxyl groups present in the crystalline portions.
Processes for activating cellulose have been proposed to do this. These processes have the aim of breaking the crystalline regions of cellulose to make them amorphous and accessible to substitution compounds and to make it possible to carry out a substitution of the hydroxyl groups which is total or partial but distributed homogeneously.
Activating agents are generally used simultaneously with the substitution compounds and are solutions of hydroxides such as metal hydroxides, for instance sodium hydroxide, aqueous ammonia, amines, dimethylformamide, dimethyl sulphoxide, acetic acid and quaternary ammonium hydroxides. The activating agent most frequently used is sodium hydroxide, which can also act as a catalyst in etherification reactions.
In order to obtain high activation and thus a dislocation of the crystalline portions which is as complete as possible, large amounts of activating agent are used. Consequently, these processes require steps for purifying the cellulose derivatives which are an economic burden on the preparation processes and which may partially explain the limited industrial and economic development of cellulose derivatives with the exception of a few.
Thus, the discovery of novel processes for manufacturing these derivatives more economically and with a higher degree of purity might allow a cost-effective development of these products.
One of the aims of the present invention is to overcome this problem by proposing a reactive cellulose, i.e. a cellulose having, in particular, a very low degree of crystallinity which can be obtained with a high degree of purity. This novel reactive cellulose is useful in particular as a starting material in the manufacture of cellulose ethers.
To this end, the invention proposes a reactive cellulose substituted with organic groups to a degree of substitution DS of less than 0.2, advantageously between 0.04 and 0.2.
The degree of substitution DS in the cellulose industry is defined as the average number of substituted hydroxyl groups per unit of glucose anhydride. As each glucose anhydride unit comprises three accessible hydroxyl groups, the maximum degree of substitution DS is equal to 3.
According to one preferred characteristic of the invention, the cellulose of the invention has a degree of crystallinity of less than 10%.
Thus, the cellulose of the invention no longer comprising crystalline portions, or only a very small proportion thereof, will make it possible to obtain cellulose derivatives without the need for a concomitant, contaminating activation step. Moreover, the cellulose derivatives obtained using the reactive cellulose of the invention have a better distribution of the substituents in the cellulose, this more homogeneous distribution being reflected by an improved solubility of the cellulose derivatives for a lower degree of substitution.
According to another preferred characteristic of the invention, the organic groups substituted with hydroxyl groups are, in particular, hydrocarbon-based groups which can comprise hetero atoms. Thus, organic groups which may be mentioned are:
linear or branched alkyl radicals comprising from 1 to 6 carbon atoms,
aryl, alkylaryl and arylalkyl radicals,
alkyl radicals comprising polarizing functions such as a carboxyl, nitrile or hydroxyl function.
Examples of organic groups which may be suitable for the invention are:
methyl, ethyl, propyl, benzyl,
hydroxyalkyl, such as hydroxypropyl, hydroxyethyl,
carboxymethyl, cyanoethyl, sulphoethyl.
Needless to say, the celluloses of the invention can comprise organic substitution groups of varied nature.
A subject of the invention is also processes for manufacturing the reactive celluloses described above.
These processes consist in treating a cellulose obtained using natural cellulose from plant sources such as wood, cotton, flax, China grass, jute, certain algae, waste from the agrifood industry, or from animal, bacterial, fungal or amoebal sources.
These natural sources of cellulose are treated with concentrated basic solutions to remove the hemicellulose and to recover a cellulose of suitable purity.
According to the invention, the cellulose thus isolated is subjected to an activation step by treatment with an activating agent to make the hydroxyl groups to be substituted accessible, followed by reacting this activated cellulose with at least one substituting agent, such as an etherification or esterification agent, and finally recovering the modified reactive cellulose.
In a first embodiment of the process of the invention, the activated cellulose is subjected to a step of partial removal of the activating agent before mixing it with the substituting agent. Advantageously, the residual weight content of activating agent after this removal step is less than 10%.
This removal is carried out either by washing or by evaporating the activating agent, or by entrainment, for example by washing with a solvent for the activating agent, in which the cellulose is insoluble.
This step for removing the activating agent makes it possible, particularly when the activating agent is an alkaline solution, to remove a large proportion of this agent and give an amorphous cellulose which is not contaminated with the said agent.
The activation can be carried out with pure liquid ammonia placed in contact with the cellulose to be activated, under high pressure and temperature, followed by activation of the cellulose either by abrupt pressure reduction in the closed chamber containing the ammonia/cellulose mixture, or by extraction or suction of the ammonia-impregnated cellulose and abrupt depressurization of the said cellulose. These two activation processes are described, respectively, in patent applications DE 19511061 and WO 96/30411.
The removal of the ammonia in the activated cellulose is advantageously carried out by evaporation to give a residual NH3 content of less than 2%.
This activation can also be carried out by treating the cellulose with an alkaline solution such as sodium hydroxide according to a process known as xe2x80x9cmercerizationxe2x80x9d.
After activation, the cellulose is washed with a solvent for the sodium hydroxide such as methanol or ethanol, to give a weight concentration of sodium hydroxide of less than 10%, advantageously between 2% and 10%.
These activated celluloses with a depleted content of activating agent are subjected to a substitution reaction by adding a substituting agent, according to operating conditions which vary depending on the nature of the substituting agent.
Generally, the amount of substituting agent added corresponds to the stoichiometric amount required to obtain the desired degree of substitution (DS), and advantageously corresponds to a molar excess of less than 150% relative to the said stoichiometric amount.
This reaction can be carried out in the presence or absence of a catalyst. Thus, conventional esterification catalysts will be used in the esterification reactions.
The substituting agents are compounds comprising the organic substitution group defined above, and a function which reacts with hydroxyl groups. As reactive functions which are useful, mention may be made, for example, of carboxylic, acid anhydride, acid halide, epoxy, isocyanate and halogen functions, and activated ethylenic bonds such as acrylonitrile or vinylsulphonate functions. Carbon sulphide CS2 can also be used as a substituting agent, and leads to a cellulose xanthate. As substituting agents which are suitable for the invention, mention may be made of:
acetic anhydride (cellulose acetate),
formic acid (cellulose formate),
sodium chloroacetate (carboxymethyl cellulose),
ethylene oxide (hydroxyethylcellulose),
propylene oxide (hydroxypropylcellulose),
alkyl halide (alkyl cellulose),
benzyl halide (benzyl cellulose),
acrylonitrile (cyanoethylcellulose),
urea (cellulose carbamate),
sodium chloroethanesulphonate (sulphoethylcellulose).
In the variant of activating cellulose with ammonia, a treatment of the activated cellulose with a stoichiometric amount of sodium hydroxide corresponding to the desired degree of substitution DS is carried out prior to the reaction with the substituting agent, when the latter comprises a halide radical in the function which is to react with the hydroxyl groups.
Advantageously, the substituted cellulose obtained after reaction with a substituting agent can be subjected to a purification step such as, for example, a step of washing with water. This step is not obligatory and is carried out only if it is necessary in order to obtain the desired degree of purity.
Thus, this purification step will often be unnecessary when the cellulose activated with ammonia is reacted directly with a substituting agent.
In a second embodiment of the process for manufacturing the reactive celluloses of the invention, the substituting agent is added to the ammonia before the activation step.
This embodiment is suitable for substituting agents which are soluble in pure liquid ammonia or dispersible in pure liquid ammonia.
In addition, this substituting agent must be chemically inert with respect to ammonia.
As a substituting agent which is suitable for this second embodiment of the process of the invention, mention may be made, in addition to the substituting agents already listed, of oxazoline, for example.
The use of these celluloses with a low degree of crystallinity is not limited to the use described above, but can also comprise a use as a polymer matrix for the manufacture of compositions intended to be shaped by conventional moulding techniques such as injection or extrusion.